Compounds such as organic matter and nitrogen contained in wastewater are capable of being oxidized and transformed by bacteria which use these compounds as a food source. Typically, heterotrophic bacteria digest the organic matter while nitrifying bacteria use the non-carbon compounds as a food source, for example, oxidizing ammonia to nitrate (a process known as nitrification to those skilled in the art).
In existing systems, for example as described by the US Environmental Protection Agency Manual on Nitrogen Control (USEPA, 1993); Wastewater Engineering, Treatment and Reuse, 4th Edition (Metcalf and Eddy, 2003); Small and Decentralized Wastewater Management Systems (Crites and Tchobanoglous, 1998); and Design and Retrofit of Wastewater Treatment Plants for Biological Nutrient Removal (Randall et al., 1992), nitrifying bacteria are much more cold sensitive and as a consequence the nitrification process virtually ceases when the water temperature approaches (e.g., decreases towards) 4 degrees Celsius.
A common form of biological wastewater treatment is the sewage treatment lagoon and these lagoons typically discharge elevated levels of ammonia during winter months in regions in which the water temperatures approach 4 degrees Celsius or lower. In view of changing environmental regulations, it would be highly advantageous to develop biological treatment processes that could remove ammonia at water temperatures of less than 4 degrees Celsius.
In existing systems for removing pollution from water, a subsurface constructed wetland system may use forced bed aeration and variable water levels to establish staged anaerobic and aerobic zones within the system. While such systems may deliver oxygen to the wastewater via aeration in a system utilizing attached-growth bacteria for treatment, they cannot provide improved removal of ammonia at water temperatures approaching 4 degrees Celsius.